The present invention pertains generally to plates for holding assays. More particularly, the present invention pertains to holding plates and methods for their use that facilitate the observation, detection and retrieval of specimen samples as they are being held on the plate in a fluid solution. The present invention is particularly, but not exclusively, useful for altering the optical characteristics of fluid solutions, as they are being suspended in the through-hole wells of holding plates, for the purpose of detecting specimen samples that are being held in the solution.
Capillary action is a phenomenon associated with surface tension that occurs in fine bore tubes or channels. Typically, such tubes or channels are referred to as capillary tubes, and it is well known that the elevation to which a liquid will rise in a capillary tube can be mathematically determined. It happens that this phenomenon has many applications, one of which is that it can be used to fill the through-hole wells of a holding plate. More specifically. it has been shown that through-hole wells having aspect ratios greater than about 5:1, and inner diameters that are less than approximately five hundred microns will exhibit the capillary phenomenon.
It is a consequence of capillary action that the liquid in a fine bore tube, such as a through-hole well in a holding plate, will form a meniscus. This meniscus, which is a departure from a flat surface where a liquid meets a solid, is caused by surface tension and is easily observable. Importantly, a meniscus will refract light that is passing into or out of the liquid in a manner that is dependent on its particular shape. In the case of aqueous solutions, and most other light transmitting fluids, the meniscus will be generally concave. Accordingly, the meniscus will optically function as a concave lens that causes light entering the liquid to diverge.
One important capability of any assay holding plate is that the specimen samples that are being held in the plate are detectable and observable. in the case of holding plates that have capillary tube-like, through-hole wells, there are optical issues that need to be resolved when light is being used for these purposes. As indicated above, as light enters through a concave meniscus into a sample solution, the concave meniscus will cause the light to diverge. If the walls of through-hole wells in a holding plate are light absorptive, as may be desired, diverging light will be absorbed by the walls. This fact can significantly reduce the amount of light that is available for interaction with a specimen sample in the solution. Under these circumstances, detection of the sample is more difficult.
in light of the above, it is an object of the present invention to provide a system and method for detecting specimen samples that are being suspended in a liquid solution under surface tension in a holding plate. Another object of the present invention is to provide a system and method that creates a convex or flat meniscus on a liquid solution which will cause light entering the solution to converge and concentrate, to thereby facilitate the detection of any specimen samples that are being held in the solution. Still another object of the present invention is to provide a system and method for detecting specimen samples suspended in a solution in a capillary tube that is easy to use, simple to manufacture and comparatively cost effective.
A system for optically detecting samples in a solution, while the solution is being held in a capillary tube under surface tension, includes a holding plate that has opposed first and second surfaces. A plurality of substantially parallel through-hole wells (i.e. capillary tubes) extend through the holding plate between these opposed surfaces. As envisioned for the present invention, the holding plate can have more than one thousand such through-hole wells, and each through-hole well will have an aspect ratio that is greater than about 5:1. Also, each through-hole well will have an inner diameter that is less than approximately five hundred microns.
A pneumatic pump, or some similar type device well known in the pertinent art, is engageable with the holding plate to establish a differential pressure (xcex94p) between the first and second surfaces of the holding plate. This differential pressure (xcex94p) will, of course, also affect any liquid solutions that are being held in respective through-hole wells of the holding plate. The result of this is that the differential pressure (xcex94p) tends to force the liquid solutions from their respective through-hole wells. in this case, the force reacting against the differential pressure (xcex94p) will be primarily the result of surface tension on the liquid solution. Mathematically, a surface tension calculation for a 200 xcexcm capillary can be made using the expression:
P=4"sgr"/d
where
P=pressure due to meniscus
"sgr"=water surface tension=0.0727 N/m
d=capillary diameter=200 xcexcm
the result will be
P=(4xc3x970.0727)/(200xc3x9710xe2x88x926)=1454 N/m2≈0.2 psi.
Thus, by properly controlling the differential pressure (xcex94p) to less than approximately two tenths of a pound per square inch (0.2 psi), the liquid solution can be moved through the through-hole well, but not forced from the well. Instead, as the fluid solution attempts to leave the through-hole well, it will bulge at the exit to form a convex meniscus. This effect can be further enhanced by coating the surface that surrounds the entrance/exit of the through-hole well with a hydrophobic coating, such as Teflon(copyright).
As intended for the present invention, the convex meniscuses that are created at the entrances, or exits, of respective through-hole wells on a surface of the plate are used to optical advantage. Specifically, when lighting devices are used to detect samples held in the through-hole wells of a holding plate, these convex meniscuses will cause light that passes into the solution through the meniscus to converge, rather than diverge. The resultant concentration of light in the solution can then be used to facilitate optical detection of samples in the solution. It will be appreciated by the skilled artisan that this result can be at least partially achieved merely by moving the bolus of fluid closer to the exit/entrance of a through-hole well.